1. Field of the Invention
The present invention relates to low-viscosity polyurethanes containing acryloyl groups, which may be converted into cross-linked products by radiation hardening. The invention also relates to a method for their preparation and to their use as binders for coating compositions.
2. Description of the Prior Art
Polyurethanes which contain acryloyl groups are known and may be obtained inter alia as the products of the reaction of polyisocyanates and hydroxyalkyl acrylates as described in DE-OS 1,644,798, DE-OS 2,115,373 or DE-OS 2,737,406. They are hardened by means of high-energy radiation such as UV or electron beams and may be employed for coating wood or for the production of coatings for graphics purposes.
A wide range of different polyisocyanates may be used to prepare urethane acrylates. If the primary interest is the production of lacquer films which are resistant to yellowing, (cyclo)aliphatic polyisocyanates are employed for the preparation of the urethane acrylates. Although hexamethylene diisocyanate is also mentioned among other materials in DE-OS 2,115,373 as being a suitable diisocyanate starting material, this diisocyanate cannot be used in the formulations described in this prior publication due to the crystallinity of the urethane acrylates produced, which render them unsuitable for use as a lacquer vehicle.
DE-OS 4,027,743 describes urethane acrylates based on hexamethylene diisocyanate and modified with ester alcohols, and which after their preparation are liquid at room temperature. However, these products, which are of high to medium viscosity, become turbid at temperatures less than 10.degree. C.
An object of the present invention is to provide low viscosity, aliphatic polyurethanes containing acryloyl groups, which are prepared from hexamethylene diisocyanate (HDI) and which are liquid at temperatures of less than 10.degree. C. without additional solvent having to be employed.
This object may be achieved with the aliphatic polyurethanes containing acryloyl groups according to the invention and their method of preparation, which are described in detail hereinafter.